Treatment of chronic humoral rejection is a major challenge in contemporary organ transplantation. This rejection process is associated with progressive arteriosclerosis which culminates in graft failure. Research performed over the last 8 years in our laboratory has demonstrated that this form of rejection involves chronic humoral and cellular immune responses directed at donor HLA antigens. Because patients with chronic rejection develop T helper cell dependent IgG alloantibodies, we have postulated that the key pathogenic mechanism resides in the activation of the indirect pathway of T cell alloreactivity. T cells engaged in this pathway recognize the alloantigen only following processing and presentation by host APC. We have shown that T cells recognize a single dominant epitope of an allogeneic HLA-DR molecule and that the response is oligoclonal. The long-term goal of this application is to suppress the indirect recognition pathway and induce alloantigen specific T cell unresponsiveness. We propose to explore three integrated strategies to achieve this goal. The first strategy consists of engineering inhibitory peptides with structural homology to the dominant T cell alloepitope, by modifying major T cell contact residues. The second strategy is based on the induction of high zone tolerance using allopeptide agonists alone or complexed with soluble HLA-DR molecules of host origin. These two strategies may result in alteration of signaling pathway leading to anergy, change in lymphokine production ,and/or apoptosis. The inhibitory capacity of the antagonist and agonist reagents, from these studies will be analyzed in three settings: a) in vitro, using allopeptide-specific T cell lines and clones; b) ex vivo, using T and B cells from patients undergoing chronic rejection and c) in SCID mice, transplanted with patient's lymphocytes. In each instance the read-out will be the inhibition of The dependent-anti-DR specific antibody production. The third strategy is based on our finding that TCR usage in indirect recognition is limited and biased by the host HLA molecule which presents the dominant alloepitope. We will use synthetic peptides, derived from the structure of an "allopeptide-specific TCR", to elicit regulator T cells and we will characterize their specificity and functional attributes. The in vivo relevance of such TCR-specific regulator cells will be investigated by testing T cells from transplant recipients and correlating the anti-TCR response with the function of the graft. The results of these experiments will establish the validity of our hypothesis that chronic rejection involves a balance between attacking and regulatory T cells and that manipulation of these cells may permit maintenance of long-term tolerance.